1. Field of the Invention
The present invention generally relates to growth by metal organic vapor phase epitaxy (MOVPE) of semiconductor devices and, more particularly, to an improved method of reducing residual or unintentional sheet charge occurring in metal organic vapor phase epitaxy (MOVPE) grown semiconductor devices, and semiconductor devices in general and low noise high electron mobility transistors (LN HEMT) and other charge or leakage sensitive devices, in particular, produced by such improved method.
2. Description of Related Art
High electron mobility transistors (HEMTs), and particularly low noise high electron mobility transistors (LN HEMTs), are used for microwave and millimeter low noise amplifiers, as well as other RF applications. These components have been used in various environments such as radar, satellites, and cellular telephones. Growing lattice-matched and pseudomorphic materials for HEMT devices and other charge sensitive devices has continued to become of more interest through the use of metal organic vapor phase epitaxy (MOVPE) and molecular beam epitaxy (MBE).
As is recognized in the art, however, the production of HEMT by MBE generally is more expensive than production by MOVPE. In particular, the cost differential results from the fact that current MOVPE commercial reactors allow for the growth of more HEMTs at one time than MBE reactors. Commercially available MOVPE reactors now handle up to about 20 wafers or substrates at a time while MBE reactors only handle up to about 5 wafers or substrates. Therefore, in the context of larger scale production, cost becomes a significant issue when comparing the benefits of MOVPE versus MBE. As such, there has been greater interest in MOVPE than in MBE.
Conventional methods of processing using MOVPE, however, have presented performance issues which have been associated with a residual or unintentional charge at the substrate-epilayer interface. The conventional wisdom for growing InP-based and GaAs-based materials by MOVPE has been to use growth temperatures of approximately 625 degrees C. or higher. Conventional thinking has been that higher quality material is obtained in this temperature range than at lower temperatures. The residual surface charge creates n-type conduction in the epilayers which, in turn, affects the pinch-off voltage, transconductance (G.sub.m), the unity current-gain cutoff frequency (f.sub.T), and the maximum frequency of oscillation (f.sub.max) of the device. An increased residual charge causes larger pinch-off voltages. Likewise, the transconductance and unity current-gain cutoff frequency degrade with increasing interface charge, and the degradations result in lower frequency, higher noise, and higher power consumption in the device.
In addressing the need to minimize the residual sheet charge, an iron doped buffer layer has previously been grown directly adjacent to the substrate. The Fe doping creates deep levels which offset the effects of shallow donors that otherwise result in n-type conduction. The buffer layer is then semi-insulating. However, the Fe dopant is itself an undesirable contaminant which can migrate into the device layers. Also, the buffer layer has previously been made relatively thick (around the order of 3,000 .ANG.) to isolate the device from the effects of the residual sheet charge. However, the need for a buffer layer, and particularly a thick buffer layer, tends to increase costs and production time of the HEMT.
As can be seen, there is a need for improved methods of MOVPE which lower the residual charge and, hence, provide higher quality material with lower production time and costs. There is also a need for improved lattice-matched or pseudomorphic material for HEMT devices which are thinner than the current art and have a lower residual surface charge and thereby better performance characteristics.